Differentiation without cleavage has been previously described at the cellular level under a variety of conditions. None of these reports has dealt with the possibility of DNA synthesis and gene activation in the absence of cell division. The present progress report proposes to continue investigations of the kinds of RNAs and proteins produced during cleavage arrest in fertilized sea urchin eggs. The cleavage arrest is induced by cold temperatures or by ethanol treatment. Hybrid embryos have been employed to provide markers for paternal protein (primarily histone), and thus serve as indicators of the zygote genome. Arrested embryos hatch and produce cilia upon return to normal temperatures or normal sea water without ethanol. Studies of protein synthesis and transcription in these embryos are also contemplated, as are ultrastructural studies. These latter studies will be primarily concerned with the development of nucleoli in arrested eggs, and in the assembly of cilia in eggs released from cleavage arrest.